you would have to convince those who already own bitcoins to switch over to software that recognizes your fork. your fork makes each bitcoin owned to become worth less than it would be without your fork.

yes, a new version of the client can implement a change that previous clients wouldn't recognize.

transactions from the new client will only be relayed by nodes that recognize the new rules. those transactions will only be included in a block by miners who accept them as valid.

if i never change my client, i can spend on your nodes (my coins stay valuable), but you can never spend on those nodes that won't adopt your changes (your bitcoins become less valuable). greshams law says you won't get very far.

Every change made to the official client is publicly available on Github. Many people, including myself, look at each change before upgrading. If someone tried to sneak in something nasty, people would most likely know about it and would simply create a fork of the project that stayed true to the original system.

let's say the author of the official client got hired by google and decides to put in some interesting stuff.

oh darnnit, never thought of that. why am i so stupid? sell everything!

/snark

your change violates fungibility. in other words, your bitcoins would never become the same as my bitcoins.

it wouldn't be in my self-interest to adopt the change that you describe and success for your idea requires each of those holding bitcoins to act against one's own self. i cannot imagine a scenario where this would happen thus i don't fear the possibility (and likelihood) that what you describe will be attempted by someone, somewhere at some point.

You can copy the block chain at any point in time and start running it with modded clients on a different port. At this point you would have "mirrored" funds, both existing in the regular block chain, and the one you copied and are running separately. That technically wouldn't get in the way of your proposal, since it implies that the user base wants to move to Bitcoin "2.0".

Nevertheless the proposition seems odd. If Bitcoin ends up with a massive flaw and it isn't inherent to the block chain architecture, then it would naturally be patched over, without the need to jump onto another block chain. The block chain is simply a data structure protected with proof-of-work calculations. It doesn't hold any rule whatsoever in its data. If the block chain is somehow corrupted beyond retrieval or usability, you'd have to give it up altogether, and at this point starting over makes sense, but the data from the previous chain is lost to all.